Playback over WASAPI must match the sample rate of the output device. Thus applications must supply their own resampling if the playback rate does not match. Most applications don't do this but just employ MME or DirectSound, which provide transparent resampling for non-matching rates.

Since some of my content is 44.1 kHz and some 48 kHz I used to set the output rate to 192 kHz so that all audio had to be upsampled. With good software resampling and a cheap onboard codec this could, in theory, even improve the overall result. But the opposite was true. Today I couldn't longer ignore the impression that the resampled output sounded somewhat muffled and did some measurements.

You can view the result here. The upsampled output (44.1 -> 192 kHz) suffers from quite a HF roll-off in comparison to pure 44.1 kHz output.

In Windows XP the SRC quality could be adjusted. I can't find anything comparable in Windows 7. Does anyone know more?